Conventional electric batteries installed in electric vehicles, such as electric and hybrid cars, for driving the vehicles are charged with power sources outside the vehicles. In other words, domestic power sources are used to charge the electric batteries of the electric vehicles. In such electric vehicles, the electric batteries can be readily charged through charging cables connecting on-board chargers with outlets of the external power sources.
The performance of electric batteries varies depending on the temperature of the environment of usage. For example, electric batteries at low temperature can supply lower power than those at ordinary temperature. The same phenomenon also occurs in the charging characteristics, i.e., the charge currents to electric batteries at low temperature hardly increase in some cases. In a technique to solve these problems, an electric battery provided with a thermosensor, an electric heater, and an air-conditioning unit is charged at a temperature controlled within a predetermined range.
For example, the temperature of an electric battery is detected upon the start of a vehicle in a cold environment. If the detected temperature is lower than a predetermined temperature, the electric battery is warmed to a higher temperature and then is charged. The electric battery can be warmed by an electric heater, or the internal heat generated during the charge-discharge cycles of the electric battery (refer to Japanese Unexamined Patent Application Publication No. 2000-040536).
Unfortunately, the charging and discharging characteristics of the electric battery significantly deteriorate, for example, at a cryogenic temperature (deep freeze temperature) substantially below 0° C., as described above. It thus takes a long time to warm the electric battery by the Joule heat generated during the charge-discharge cycles of the electric battery. In addition, the electric battery charged at a cryogenic temperature readily deteriorates and may have a shorter service life.
The deterioration of the electric battery can be prevented through the electrical disconnection of the cold electric battery from an external power source and the warming of the electric battery by the electric heater alone. Unfortunately, the electric battery is disconnected from a circuit for supplying power (external power) from the external power source, and thus cannot provide a buffering effect (effect of reducing fluctuations in the voltage). In other words, the voltage fluctuations in the external power source directly affect an inverter or a converter for converting the external power into low-voltage DC (Direct Current) power, resulting in an error in their operations.